1. Field of the Invention
This invention relates to a burner for a vapour deposition process, in particular for a vapour deposition process for the manufacturing of a glass preform suitable for drawing an optical fibre.
2. Description of the Related Art
As is well known, a glass preform suitable for drawing an optical fibre can be produced by one of the following methods of depositing chemical substances on a deposition substrate: MCVD (Modified Chemical Vapor Deposition), OVD (Outside Vapor Deposition) and VAD (Vapor-phase Axial Deposition). In these techniques, a burner is used to generate a flame and to eject toward the deposition substrate predetermined substances that react in the flame to form deposition substances.
The preform so formed is subsequently positioned vertically within a furnace and an optical fibre is drawn from the lower part of the preform.
For manufacturing a large number of high quality optical fibres having the same characteristics, it is important to guarantee a high repeatability in the deposition process. For example, it is important that a same burner used for successive deposition processes, or different burners used in parallel for a same deposition process or for contemporaneous deposition processes, can always provide the same performances.
Advantageously, burners comprising different components that can be disassembled to allow maintenance and cleaning operations can be used. The Applicant has observed that each time one such burner is disassembled, the successive assemblage operation can give rise to misalignment problems among the different components. These misalignments can result in structural imperfections of the burner and variations in its functionality. Moreover, the Applicant has noticed that also a burner that has been correctly assembled can show misalignments during functioning, for example due to vibrations. Therefore, high process repeatability cannot be guaranteed.
It has also been rioted that, after a burner has been used in several deposition processes, seizing up of different parts of a burner can occur, thus preventing disassembly of the burner. European Patent EP0978491 relates to a precision burner for oxidizing halide-free, silicon-containing compounds. This patent tackles the problem of variability in the properties of burners used in OVD processes, which result in a variability in the preforms produced using such burners. In particular, EP0978491 describes a prior art burner, also represented (in an exploded view) in FIG. 1 of the present patent and indicated with 10, which includes:                a back block 16, defining a gas-receiving block, connected to reactants feed lines (not shown);        a face block 12 (burner head), defining a gas-ejecting block, having reactants exit holes;        a couple of manifolds 14, 20 interposed between back block and face block; and        a separate fume tube 18 that extends through back block, manifolds and face block, and adapted to convey a reactant comprising silica up to a central exit hole of the face block.        
Bolts 30, only one of which is shown in FIG. 1, are inserted through aligned bolt holes 24, 26 and 28 of back block 16, manifold plate 14 and face block 12, respectively, and tightened to hold burner 10 together.
In practice, it has been found that the positioning of inner shield manifold 20 on fume tube 18 and the alignment of face block 12, manifold plate 14, back block 16, and fume tube 18 can vary from burner to burner.
To solve these problems, EP0978491 proposes a burner with some relevant differences with respect to the above-described burner, in particular a burner having a better alignment of fume tube within back block and face block and easier to be disassembled. In particular, fume tube, back block and face block are shaped so that fume tube can be press fit into back block until it reaches a shoulder therein and so that face block can have a slide fit over fume tube. Also, in order to eliminate removable inner shield manifold 20, which was a source of substantial burner-to-burner variability for prior art burner 10, the orifices that serve to equalize the pressure of the innershield gas are formed as an integral part of fume tube.
According to the Applicant, these solutions are not completely satisfactory, since there is still the need to fit together different pieces by pressing or sliding, and both manufacturing tolerances and wear can lead to misalignment and seizing problems.
The Applicant has in particular noted that a burner like the one proposed in EP0978491 can still be subjected to misalignment between fume tube and face block, which can significantly worsen the uniformity of gas flows and the effectiveness of the reactions taking place among the different gas.
The Applicant has also observed that the same problems may be present in other burners known in the art, like the ones described in U.S. Pat. No. 4,474,593 and in U.S. Pat. No. 3,698,936, wherein there is a separate central tube that fit into other parts of the burner for defining a central gas passage.
The Applicant has tackled the problem of providing a burner for a vapour deposition process wherein said misalignment and seizing problems can be eliminated or at least significantly reduced.